This invention relates to marine seismic surveying and, more particularly, to methods and apparatus for enhancing marine seismic profiles to minimize destructive effects of coherent noise, e.g., that caused by water column reverberation, or "ghosting."
Generally speaking, marine seismic surveys are conducted by towing an energy source and seismic detector behind a vessel. The source imparts an acoustic wave to the water, creating a wavefield which travels coherently into the underlying earth. As the wavefield strikes interfaces between earth formations, or strata, it is reflected back through the earth and water to the detectors, where it is converted to an electrical signal and recorded. Through analysis of this signal, it is possible to determine the shape, position and make-up of sub-bottom formations.
A problem encountered in marine surveying--as well as in its land counterpart, vertical seismic profiling, or "VSP"--is that of water column reverberation. The problem which arises as a result of inherent reflectivity of the water surface and bottom may be explained as follows.
A seismic wave generated in (or reflected off) earth strata passes into the water in a generally upward direction. This wave, termed the "primary," travels through the water and by the seismic detector, which records the passing. The wavefield continues upward to the water's surface, where it is reflected downwards. This reflected, or "ghost," wavefield travels back through the water and by the detector, where its passing is again recorded. Depending upon the nature of the earth material at the water's bottom, the ghost wavefield may, once again, be reflected upwards, giving rise to a series of subsequent ghost reflections. This reverberation of the wavefield obscures the primary data, amplifying certain frequencies and attenuating others, and thereby makes difficult analysis of the underlying earth formations.
Among the art is Ray et al., U.S. Pat. No. 4,353,121. That patent teaches a system in which seismic waves travelling through the water are detected by a slanted cable of spaced-apart hydrophones. After conventional data processing techniques (e.g., demultiplexing, gain recovery, common depth point sorting) have been applied to the wavefield data, static and dynamic time corrections are separately applied to the primary and ghost reflections contained therein. Particularly, the primary reflection wavelets are aligned with one another in one stack, and the ghost reflection wavelets are aligned in another. The ghost reflection data is, additionally, phase-reversed and time-shifted for correspondence with the primary reflection data. The resultant primary stack and ghost stacks are combined to produce a stack which is said to be of improved signal quality.
Also in the art is Ruehle, U.S. Pat. No. 4,486,865, disclosing a technique for reducing ghosting wherein a pressure detector and a particle velocity detector are positioned in close proximity to one another in the water. The output of the detectors to upward and downward travelling waves are gain-adjusted and deconvolved using a filter computed by adding a predetermined amount of white noise to the zero lag of the autocorrelation function. The deconvolved/gain-adjusted output of the detectors are then added to one another to cancel ghost reflections.
Prior techniques of the type described above suffer various drawbacks. For example, the output of the Ray et al system is data-compressed (i.e., stacked) and, therefore, not suited for further processing by traditional pre-stack techniques, i.e., time, amplitude and frequency correction, which might otherwise improve the quality of the seismic reflection analysis. The Ruehle patent, and others like it which utilize filtering techniques, suffers losses in signal quality inherent to the filtering process, for example, amplification of noise in certain frequencies.
In view of the foregoing, an object of this invention is to provide an improved system for marine seismic reflection prospecting. More particularly, an object of the invention is to provide a method and apparatus for marine seismic prospecting having an output with improved signal-to-noise ratios.
A further object of the invention is to provide a marine seismic reflection prospecting system which eliminates ghosting without increasing the noise content of the output signal.
Still another object of the invention is to provide a marine seismic prospecting system that reduces both coherent noise, e.g., ghosting and horizontally travelling noise, and non-coherent noise, e.g., random noise.
A further object of the invention is to provide a marine prospecting system which produces output which may be used in conjunction with other pre-stack techniques, e.g., normal moveout correction, to facilitate analysis of the earth formations under study.